Phase 2 Proposal Instructions for Cycle 10

10.6 Mode = TIMEŠTAG
Configuration = STIS/FUVŠMAMA,
Configuration = STIS/NUVŠMAMA

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TIMEŠTAG Mode is used for imaging and spectroscopy with high time resolution. When used in TIMEŠTAG Mode, the MAMA produces an event stream with a time resolution of 125 microseconds. If the incoming event rate exceeds about 770,000 counts per second, not all events get registered due to finite electronic response time. The AXIS1 and AXIS2 half-pixel coordinates (11 bits each) of each photon event, along with 9 bits of fine-resolution time data, are stored as a 32-bit word in data buffer memory. See the discussion of TIMEŠTAG Mode in the STIS Instrument Handbook.

10.6.1 Aperture or FOV

Table10.4: Supported STIS Apertures gives the list of apertures that are Supported for STIS use.

10.6.2 Spectral Element

See Table10.1: Supported Optional Parameters for STIS Instrument Modes.

10.6.3 Wavelength

Enter the value of the central wavelength in Angstroms. Table 10.2 gives the allowed values of the central wavelength for each spectroscopic spectral element. Consult the STIS Instrument Handbook for the associated minimum and maximum wavelength and other information pertaining to the spectroscopic elements.

No wavelength should be specified if the MIRROR is the spectral element.

 

10.6.4 Optional Parameters

See Table10.1: Supported Optional Parameters for STIS Instrument Modes for a listing by operating mode of all Optional Parameters. Here are descriptions for those Optional Parameters for MAMA TIME-TAG mode.

BUFFERŠ TIME
=40 or greater (integer seconds)

A required parameter which specifies the estimated minimum time in which 2 x 10^6 photon events (half of the STIS data buffer capacity) will be accumulated during a TIMEŠTAG exposure.

If the predicted total number of events from a TIMEŠTAG exposure exceeds the STIS data buffer capacity of 4 x 10^6 photon events, data must be transferred to the HST onboard science data recorder during the exposure. Transfers of data from the STIS buffer during an exposure will be made in 8-MByte blocks (half the buffer capacity). The value of BUFFERŠTIME should be the half-buffer capacity (2 x 10^6 counts) divided by the estimated maximum average count rate in photons per second (see the STIS Instrument Handbook). If the actual count rate is less than expected and fewer than 2 x 10^6 counts are received during this time, the 8-MByte blocks will contain fill data.

Note that BUFFERŠTIME should include expected counts from the detector dark current as well as the detected photon events, factoring in the instrument quantum efficiency. A conservative value of BUFFERŠTIME is recommended (err slightly on the low side) to avoid data loss. For more details, see the STIS Instrument Handbook.

10.6.5 Number of Iterations and Time Per Exposure

Number_Of_Iterations must be 1 in this mode.

Time_Per_Exposure must be an integral multiple of 0.1 second. If it is not its value will be truncated down to the next lower integral multiple of 0.1 sec. Enter the total time of data collection as Time_Per_Exposure. An exposure time of less than 0.1 sec will be set to the minimum MAMA exposure time of 0.1 second. The maximum permissible Time_Per_Exposure for this mode is 6553.5 seconds.

If BUFFERŠTIME < 99 seconds, photon events will be generated faster than data can be transferred out of the buffer during the exposure. In this case, Time_Per_Exposure should be less than or equal to 2 * BUFFERŠTIME so that the exposure can complete before data transfer is necessary. A BUFFERŠTIME of 99 seconds corresponds to a maximum average count rate of 20,200 counts/sec.

Note that TIMEŠTAG exposures have the potential to rapidly use up the HST onboard storage capacity. Caution is advised on any exposure with an exposure time greater than 30 * BUFFERŠTIME, which corresponds to 6 x 10^7 counts, or about 2 GBits (close to 20% of the solid-state recorder capacity). Please see the STIS Instrument Handbook.

10.6.6 Special Requirements

The special requirements RT ANALYSIS and PARallel WITH are not permitted on TIMEŠTAG Mode exposures.